The battery packs for portable power tools, outdoor tools and certain kitchen and domestic appliances may include rechargeable batteries, such as lithium, nickel cadmium, nickel metal hydride and lead-acid batteries, so that they can be recharged rather than be replaced. Thereby a substantial cost saving is achieved.
Some chargers can be connected to a vehicle battery, such as a car battery. Referring to FIG. 1, car battery 1 can be connected to charger 20 via a lighter plug 5. Charger 20 in turn charges battery pack 10.
Two virtual resistors 3, 4 may exist between car battery 1 and charger 20. Virtual resistors 3, 4 represent the inherent resistance before and after the lighter plug connection, which in turn create voltage drops. Accordingly, the voltage VIN received by the charger 20 may not necessarily be equal to the voltage of car battery 1.
A fuse 2 may also be provided between car battery 1 and charger 20. Typically, such fuse 2 has a rating of about 8 amps. In other words, if the current IIN going to charger 20 is larger than about 8 amps, the fuse 2 will open.
This could be problematic as charger 20 typically sends an effective constant current IOUT to battery pack 10. Such problem arises because of the following equation:(VIN)(IIN)k=(VPACK)(IOUT),                 where VIN, IIN, and IOUT are defined above,        k is the charger efficiency constant, and        VPACK is the voltage of battery pack 10.        
Under such equation, since VPACK is set by the battery pack, and IOUT as set by the charger and the charger efficiency constant k are relatively constant, the only two variables remaining are VIN and IIN. If VIN drops below a certain threshold, IIN will have to increase to maintain the equation. However, if IIN increases beyond a certain threshold, it will force fuse 2 to open, thus prematurely ending charging.